Comparisons of Photoinduced Oxidative Addition of B−H, B−B, and Si−H Bonds at Rhodium(η5-cyclopentadienyl)phosphine Centers

Abstract

Ultraviolet irradiation of [Rh(η5-C5H5)(PMe3)(C2H4)] (1a), [Rh(η5-C5H5)(PPh3)(C2H4)] (1b), and [Rh(η5-C5H4CF3)(PMe3)(C2H4)] (1c) (collectively abbreviated as [Rh(Cp‘)(PR3)(C2H4)]) in the presence of HBpin (pinacolate = pin = 1,2-O2C2Me4) results in elimination of C2H4 and B−H oxidative addition, leading to the formation of boryl hydride complexes [Rh(Cp‘)(Bpin)(H)(PR3)]. Complete conversion is achieved in liquid HBpin or by photolysis in hexane at −10 °C. Similarly, photolysis of 1a−c in the presence of B2pin2 in hexane at −10 °C leads to B−B oxidative addition products, [Rh(Cp‘)(Bpin)2(PR3)]. Irradiation at room temperature leads to formation of [Rh(Cp‘)(PR3)2] in addition to the desired products. The rhodium boryl products were characterized by multinuclear NMR spectroscopy and, in the case of [Rh(η5-C5H5)(Bpin)(H)(PPh3)], by X-ray crystallography. The structure reveals a Rh−B distance of 2.0196(15) Å. The H···B separation of 2.09(2) Å together with the bond angles at the metal suggest some residual H···B interaction. Photolysis of 1a−c in the presence of tertiary and secondary silanes (HSiEt3, HSiiPr3, HSi(OMe)3, HSiMe2Et, HSiMeEt2, and H2SiEt2) results in rhodium silyl hydride complexes [Rh(Cp‘)(SiR‘2R‘ ‘)(H)(PR3)]. The structure of [Rh(η5-C5H5)(SiiPr3)(H)(PMe3)] was determined by single-crystal X-ray diffraction, yielding a Rh−Si bond length of 2.3617(3) Å and a Rh−H bond length of 1.508(17) Å. The H···Si distance of 2.278(17) Å and the very unequal H−Rh−P and H−Rh−Si angles suggest some residual H···Si interaction. Competition reactions were performed with 1b dissolved in hexane in the presence of HBpin and B2pin2 simultaneously. 31P NMR measurements, made after brief irradiation, showed a slight preference for B−B oxidative addition over B−H oxidative addition. Similar experiments with three-way competition among HBpin, HSiMe2Et, and HC6F5, analyzed by 1H NMR spectroscopy, showed negligible selectivity among H−B, H−C, and H−Si oxidative addition. Molecular structures were also determined by single-crystal X-ray diffraction for 1b, 1c, [Rh(η5-C5H5)(PPh3)2], and [Rh(η5-C5H5)Cl2(PPh3)].